Loops in Java

Loops let a program execute a block of code multiple times without rewriting it. Java offers three main loops — for, while and do-while — plus an enhanced for-each loop for arrays and collections. Loops form the backbone of algorithms ranging from simple counting to complex data processing.

• for (init; condition; update) { /* body */ }
• while (condition) { /* body */ }
• do { /* body */ } while (condition);
• Use break to exit early, continue to skip an iteration.

Let's start with a hands-on example. The program below shows the core idea behind Loops in just a few lines of Java. Read it line by line and observe how Java executes each statement in order.

class Main
{
    public static void main(String args[])
    {
        for (int i = 1; i <= 5; i++)
        {
            System.out.println("Count: " + i);
        }
    }
}

The following examples progress from simple to more practical patterns. Try each in your IDE, change the inputs and observe the output. This is the fastest way to internalize the concept.

class Main
{
    public static void main(String args[])
    {
        int n = 10, sum = 0;

        for (int i = 1; i <= n; i++)
            sum += i;

        System.out.println("Sum = " + sum);
    }
}

class Main
{
    public static void main(String args[])
    {
        int n = 5, i = 1;

        while (i <= 10)
        {
            System.out.println(n + " x " + i + " = " + (n * i));
            i++;
        }
    }
}

class Main
{
    public static void main(String args[])
    {
        int count = 0;

        do {
            System.out.println("Run " + count);
            count++;
        } while (count < 3);
    }
}

class Main
{
    public static void main(String args[])
    {
        for (int i = 1; i <= 10; i++)
        {
            if (i == 5) break;
            System.out.println(i);
        }
    }
}

class Main
{
    public static void main(String args[])
    {
        for (int i = 1; i <= 6; i++)
        {
            if (i % 2 == 0) continue;
            System.out.println(i);
        }
    }
}

class Main
{
    public static void main(String args[])
    {
        int arr[] = {10, 20, 30, 40};

        for (int x : arr)
            System.out.println(x);
    }
}

• An infinite loop happens when the condition never becomes false — use for(;;) or while(true) intentionally only.
• Always update the loop variable to avoid infinite loops.
• Use for-each when you don't need the index.
• Avoid changing a collection while iterating over it — use Iterator.remove().
• break only exits the innermost loop. Use labeled break to exit outer loops.

Reading is not enough — practice solidifies knowledge. Try every question below in your own editor before peeking at any solution.

• Q1. Print all even numbers from 1 to 50.
• Q2. Find the factorial of a number using a for loop.
• Q3. Reverse a number using a while loop.
• Q4. Count the digits of a positive integer.
• Q5. Print the Fibonacci series up to 20 terms.

These are the most common questions asked in Java interviews on this topic. Memorize the concept, not just the answer — interviewers often follow up with edge cases.

Even experienced developers slip on the same pitfalls. Watch out for these classic mistakes while working with Loops in Java:

• Forgetting the semicolon ; at the end of statements while using Loops.
• Mixing up similar method names — read Java docs before using new APIs related to Loops.
• Ignoring compiler warnings — they often hint at bugs that will appear later in production.
• Hard-coding values that should come from configuration files or environment variables.
• Not handling edge cases: empty inputs, very large inputs, negative numbers and null references.
• Skipping unit tests — small tests prevent big regressions, especially around control flow.